How to get selected edges length?
Update
I recommend using the newer measureit addon instead.
To measure the sum of the length of a bunch of edges like this:

Select the edges
Set the Sum group in Tool Shelf &gt; Measurit &gt; Tools
Click Segment in Tool Shelf &gt; Measurit &gt; Tools


Now the total length of those edges will be displayed at the bottom of the Measureit panel in the properties region (N).
Be aware that addons have no reliable way to track vertices, so if you modify the mesh your measurements may go awry.

Original answer
You could use the new Ruler/Protractor tool located in the 3D view &gt; Tool Shelf &gt; Ruler/Protractor

Note that this was just added in Blender 2.67.